1. Field of the Invention
This invention relates to an improved process for making methyl chloride by reacting methanol with hydrogen chloride in the presence of a metal chloride catalyst, e.g., zinc chloride. As such a process normally results in the formation of substantial amounts of dimethyl ether (DME) as an unwanted by-product, the improvement is particularly concerned with a method of reducing such ether formation. 2. Description of the Prior Art
Methyl chloride has many uses in industry and science. For instance, it not only finds wide use as a solvent, extractant, propellant or refrigerant, it is also used as a methylating agent in organic synthesis and, importantly, as a starting material in the production of dichloromethane, trichloromethane and carbon tetrachloride by reaction with chloride. Another important use of methyl chloride is as an intermediate in the production of silicones.
A common and economical process for providing methyl chloride involves the hydrochlorination of methanol according to the reaction MeOH+HCl.fwdarw.MeCl+H.sub.2 O. As it is described, for instance, in the Encyclopedia of Chemical Processing and Design, Vol. 8, pp 256-9 (1979), this reaction can be carried out using either a liquid or a solid catalyst. However, the object of this invention is concerned only with the reaction when it is conducted in the liquid phase.
The liquid-phase reaction system is usually conducted in an aqueous medium, at or near atmospheric pressure and at a reaction temperature of from about 100.degree. to about 200.degree. C. Catalysts for the liquid-phase reaction are water solutions of metal chlorides, such as ferric chloride or bismuth oxychloride or most preferably zinc chloride. The reaction has been most commonly carried out by bubbling HCl and methanol as a gaseous mixture through a single reactor or several reactors operating in parallel, although sometimes the reactors have been operated in series. In all such conventional cases, however, substantially all of the methanol and all of the HCl is fed through a single-feed system into the first and in some cases the only reaction stage. As shown in U.S. Patent 2,847,484, when more than one reactor is used, the effluent vapors are passed from the first reactor through a subsequent reactor or reactors operating in series. The temperature of the solution and the concentration of catalyst are so maintained that the water of reaction is continuously vaporized from the reaction mixture along with the methyl chloride, thereby providing continuous operation. By-product hydrogen chloride, such as that available from the chlorination of methane or ethane, can be used.
As the desired methanol hydrochlorination reaction is generally accompanied by a side reaction wherein dimethyl ether (DME) is formed from methanol according to the equation 2MeOH.fwdarw.DME+H.sub.2 O, the crude methyl chloride obtained by the hydrochlorination of methanol contains not only water, unreacted methanol, and hydrochloric acid but also a substantial proportion of DME as an impurity. The methanol and hydrochloric acid are usually removed by scrubbing the crude methyl chloride vapor with cold water and then with cold dilute sodium hydroxide solution, and the water and DME are removed by scrubbing with cold concentrated sulfuric acid. The resulting pure methyl chloride vapor is compressed, condensed with cold water, and placed in storage tanks.
While removal of DME from the methyl chloride is readily accomplished by scrubbing with sulfuric acid, not only is its formation wasteful in terms of greater raw material costs, but the disposal of unwanted sulfuric acid-containing DME presents a problem in view of increasing ecological concerns and resulting restriction on the disposal of industrial waste streams.